1. Field of the Invention
This invention relates to the art of brown coal liquefaction, especially to the art of effecting hydrogenolysis (hereinafter referred to as "hydrogenation") of pulverized brown coal in the slurry form. More particularly, it relates to a method of increasing and recovery of naphtha and other oil fractions by selecting, as the slurrying solvent, a solvent suited for high-efficiency hydrogenation and conducting the hydrogenation in two steps.
2. Description of the Prior Art
In the two-step hydrogenation process known as a method of liquefying brown coal, the first hydrogenation (hereinafter referred to as "primary hydrogenation") step comprises blending pulverized brown coal, a slurrying solvent and a hydrogenation catalyst with one another and subjecting the mixture to reaction with hydrogen at high temperature and high pressure. It has already been confirmed that the hydroliquefaction efficiency is much influenced by the adequacy for hydrogenation of the slurrying solvent used. If the slurrying solvent is inadequate for hydrogenation, the SRC (solvent-refined coal) formed by hydrogenolysis will contain structures with advanced ring condensation and, in extreme cases, will lead to the phenomenon of coking. It has also been confirmed that the use of a slurrying solvent suired for high-efficiency hydrogenation increases the recovery of oil fraction in the primary hydrogenation and the yield of fractions corresponding to the light distillate oil and further results in an increase in the proportion of benzene-soluble components as a result of promoted hydrogenation of the resulting SRC. The use of a high-grade catalyst as the hydrogenation catalyst may of course help compensate for the disadvantage of a slurrying solvent which is inadequate for hydrogenation. Due to the strong demand from the cost viewpoint, however, inexpensive iron-based catalysts, for instance Fe.sub.2 O.sub.3 +S, are generally used as the primary hydrogenation catalysts and at the same time a slurrying solvent not well-suited for hydrogenation is in use. Under these circumstances, the hydroliquefaction efficiency necessarily remains at a low level. The same also applies to the second step hydrogenation, where a high-grade molybdenum-based catalyst is generally used. In fact, however, the slurrying solvent used therein is not very adequate for hydrogenation and therefore the particular technological constitution, namely the addition of a secondary hydrogenation step, cannot produce the intended effect to the full.
The hydroliquefaction reaction is supposed to proceed in the manner of pyrolysis (thermal degradation) of brown coal to lower-molecular-weight compounds and stabilization of radicals formed thereby by reaction with hydrogen. It is also known that the yield of oil fraction given by liquefaction depends on the balance between the rate of pyrolysis and the rate of hydrogen supply. At a relatively high rate of hydrogen supply, the oil fraction yield is high, whereas a slow hydrogen supply rate results in an increase in the yield of heavy distillate oil fraction but in a decrease in the yield of oil fraction. In any case, however, in the current situation where the slurrying solvent is in itself inadequate for hydrogenation, it is natural that an increase in the hydrogen supply rate cannot directly produce the effect of increasing the oil fraction yield.
Therefore, a need continues to exist for a method of liquefying brown coal which uses a slurrying solvent which is adequate for high-efficiency hydrogenation in each of the primary and secondary hydrogenation steps, thereby increasing the efficiency of hydrogenation in each hydrogenation step, and which enables the recovery of oil fractions in good yield.